1. Field of the Invention
The present invention relates to a power supply apparatus and an image forming apparatus and, more particularly, to improving the efficiency of a power supply apparatus of ringing choke converter (hereinafter referred to as RCC) system under light-load conditions.
2. Description of the Related Art
The recent increase in consciousness about environment and the like is producing a requirement to reduce power consumption of electronic devices. In general, for example, an electronic device such as a TV set or a printer is often in the standby state waiting for a user operation for a longer time than that in an actually operating state. For this reason, to reduce power consumption, it is important to improve the efficiency of the power supply apparatus in the standby state, that is, under light-load conditions.
On the other hand, a self-excited type power supply apparatus represented by the RCC type is simple and inexpensive and is therefore mounted in various electronic devices. As a means for improving the efficiency of the RCC type of the power supply apparatus (hereinafter referred to as RCC power supply) under light-load conditions, for example, in Japanese Patent Application Laid-Open No. 2000-278946, it is proposed, a method of lowering the output voltage and intermittently operating the RCC power supply apparatus in the standby state, thereby improving the efficiency.
However, when an output voltage V0 is lowered, and an FET 404 shown in FIG. 1 to be described later is turned on via a resistor 405, as in the related art, the loss of the FET 404 at turn-on increases. This will be described below with reference to FIG. 5.
FIG. 5 is a timing chart showing waveforms when the RCC power supply apparatus of the related art (for example, FIG. 1 to be described later) is operating in an intermittent oscillation state, and illustrates the waveforms of a drain-source voltage (Vds), a drain current (Id), and a gate-source voltage (Vgs) of the FET 404 from above. As shown in FIG. 5, in the intermittent operation state, the FET 404 cannot be turned on only by the voltage generated in a winding Nb. Hence, the FET 404 is turned on when the current flowing through the resistor 405 charges the gate-source capacitance. However, to reduce power consumption under light-load conditions, in general, the resistor 405 needs to have a high resistance, and the current flowing through the resistor 405 is very small. Hence, when turning on the FET 404 via the resistor 405, the turn-on of the FET 404 is largely delayed, and the loss at turn-on increases. Additionally, at turn-on of the FET 404, charges accumulated in a capacitor 414 are consumed as the loss of the FET 404. This loss increases in proportion to the square of Vds at turn-on of the FET 404. In the intermittent operation state, since Vds at turn-on of the FET 404 is almost equal to the potential of a capacitor 403 (illustrated as C 403 in FIG. 5), the loss further increases. Due to the above-described reasons, there is a limitation in improving the efficiency under light-load conditions only by lowering the output voltage V0 and operating the RCC power supply apparatus in the intermittent oscillation state.
When the number of turns of the winding Nb is increased so as to make the voltage generated in the winding Nb higher than the gate threshold voltage of the FET 404 even when lowering the output voltage V0, the voltage generated in the winding Nb when the output voltage V0 of normal level is output becomes high. This undesirably leads to a need to raise the breakdown voltages of circuit components, resulting in an increase in the cost. In addition, an operation error may occur and break the circuit.